Metal oxides, in particular mixed metal oxides, have an ever greater field of use, in particular in ceramics, polymer additives, fillers, pigments, reactive surfaces and catalysts etc.
Additionally, for example copper oxides with a perovskite structure display phase transitions to superconductors at surprisingly high temperatures and are therefore the subject of numerous investigations. This applies in particular to the classes of the lanthanum-strontium cuprates, for example La2-xSrxCuO4 or to yttrium-barium cuprates, such as for example YBa2Cu3O7-Δ.
Typical representatives of these classes of compounds are for example the so-called titanates, zirconates and hafniates, which are divided in particular into the classes of orthotitanates M2IITiO4 and metatitanates MIITiO3.
However, these compounds almost never contain the discrete ions [TiO4]4− and [TiO3]2−, analogously to the phosphates or sulphites. The structures of these mixed metal oxides consist of three-dimensional ion arrangements which are particularly interesting.
If MII is of a comparable size to TiIV, for example in the case of M=Mg, Mn, Fe, Co, Ni, then that structure of the ilmenite, FeTiO3, is present which is constructed from hexagonally very tightly packed oxygen atoms, wherein one third of the octahedral holes is occupied by MII and a further third by TiIV. This corresponds essentially to the so-called basic structure of Al2O3, with the difference that in the latter cases there is only a type of cations which contain two-thirds of the octahedral spaces.
If, on the other hand, MII, is substantially larger than TiIV (for example M=Ca, Sr, Ba), then the structure of the perovskite, CaTiO3, is preferred.
Perovskites can be thought of as being constructed from a cubically very tight packing of spheres of calcium and oxygen atoms, in which the former are arranged regularly and the titanium atoms exclusively occupy the octahedral holes formed by the oxygen atoms, with the result that they can be kept as far away as possible from the calcium atoms. The perovskite lattice is widened by the size of the BaII ions such that the titanium atom is too small to fill all of the octahedral hole. Ferro- and piezoelectric properties are brought about by this. Barium titanate is used for example during the preparation of compact capacitors because of its high dielectric constant, as well as in ceramic transducers in the case of microphones and pick-ups.
The compounds M2IITiO4 (m=Ng, Zn, Mn, Fe, Co) adopt the so-called spinel structure of MgAl2O4. This is the third important structural type which is preferred by several mixed metal oxides. Here, the cations possess both octahedral and tetrahedral holes in a cubically very tight packing of spheres arrangement of the oxide ions.
Such mixed metal oxides, in particular for example perovskites, are also used as catalysts, for example in the field of catalytic converters in cars, during the preparation of photocatalysts and for the preparation of oxidic catalysts, in particular for the preparation of methanol and the oxidation of carbon monoxide. Here the process of calcining the starting materials during the preparation process greatly influences the quality of the end catalysts and thus also their possible uses in catalysis. (see Zuhlke, Dissertation, TH Karlsruhe 1999)
The targeted control of the crystallization process can be influenced by the composition of the educt(s). An important factor here, in particular when used in catalysis, is the crystallite size (R. Schlögel et al, Angewandte Chemie 116, 1628-1637, 2004).
Nanocrystalline “powders” are also increasingly coming into consideration, despite the fact that preparation problems have for the most part remained unsolved.
Such nanocrystalline mixed oxide powders have thus far usually been prepared either by (wet-)chemical synthesis, by mechanical processes or by so-called thermophysical processes.
In the case of perovskites, BET surface areas of approx. 2 to 10 m2/g are achieved with the conventional processes known thus far.
Typically, during the chemical synthesis of nanocrystalline powders, starting from so-called precursor compounds, a powder is synthesized by chemical reactions for example by means of hydroxide precipitation, synthesis by hydrolysis of organometallic compounds and hydrothermal processes. The definitive structure of nanocrystallites typically establishes itself, as already mentioned, only after or during calcining.
Mechanical preparation processes are characterized by intensive grinding of inhomogeneous particles into homogeneous particles, which often also leads to undesired phase transformations to the point where particles become amorphous due to the pressure exerted on the particles.
Typically, the particles formed in the process are not present in a uniformly homogeneous size distribution. Moreover, there is the risk of abrasion by the grinding tools, and thus of a contamination of the products, which is disadvantageous in particular when using the thus-obtained nanocrystalline mixed oxides in the field of catalysis.
Thermophysical methods are for example described in WO 2004/005184. These are based on the introduction of thermal energy into solid, liquid or gaseous starting compounds. The above-mentioned international patent application relates in particular to the so-called plasma-pyrolytic spray process (PSP), in which the starting materials are atomized in an oxyhydrogen flame and decomposed in the process. A preferred technical application of this technology is in the preparation of fine crystalline silicon dioxide in which readily volatile organosilicon compounds are atomized in an oxyhydrogen flame.
Moreover, during the synthesis of nanocrystalline particles the so-called plasma synthesis process has been used in which the starting materials are evaporated in a 6000K-hot plasma. Further customary processes of the state of the art are for example CVD processes in which gaseous educts are reacted, wherein non-oxidic powders or mixed oxide compounds with different phase structures also often form.
The above-named processes of the state of the art have disadvantages in particular in relation to the presence of a very broad particle-size distribution of the nanocrystallites, undesired agglomerations of the nanocrystalline particles among one another and also incomplete phase transitions, i.e. often, only 40 to 70% of the desired end-product is obtained in the end-product, which necessitates further purification steps or recrystallization.